Motor velocity is an important signal for electronic steering. It is used in the control of the motor as well as in steering related functions such as active damping and inertia compensation. In many systems, the motor velocity signal is obtained by differentiating the motor position signal or the signals used to derive motor position with respect to time. In this case, there is a tradeoff between lag, resolution and noise that can be obtained by setting the period of the differentiation or filtering the signal with a low pass filter. Increasing the differentiation period improves noise and resolution but degrades lag. Filtering with a low pass filter also improves noise and resolution at the expense of lag.
Noise and resolution of the motor velocity signal are important because they translate into audible noise and torque disturbances in the steering system. Lag is important because it directly affects the dynamic performance of the system. Excess lag is often perceived as “inertia” and is not desirable.
Many motor velocity measurement schemes exist today, including the following three schemes. The first is a discrete digital motor position sensor that is used to derive a velocity signal using a finite difference differentiation with respect to time. The second improves on the first by adding an analog tachometer for low motor speeds, blending with the differentiated position at higher motor speeds. The third method uses an analog position sensor that generates signals representing the sine and cosine of the motor shaft angle. The sine and cosine signals are filtered with a moving average oversampling filter to improve noise and resolution. The filtered sine and cosine signals are differentiated with respect to time using a finite difference differentiator. The differentiated sine and cosine signals are then processed using the angle difference identity for sine to get velocity.
These motor basic motor velocity measurement schemes have their advantages as well as limitations in their implementations. Accordingly, it is desirable to provide methods and systems that improve accuracy in measuring motor velocity.